The proposed projects are directed towards: a) an investigation of the physical and chemical properties of the carbon-cobalt bond of the corrinoid coenzymes. b) a study of the mechanism of the 5'- deoxyadenosylcobalamin-dependent reduction of ribonucleotides with suitably labeled ATP analogs. c) the purification of ribonucleotide reductases by affinity chromatography. d) the purification of the thioredoxin-thioredoxin-reductase system of Corynebacterium-nephridii. e) isolation of an inhibitor of ribonucleotide-reductase from Euglena gracilis.